Filling box-like object with random objects
You can use a particle system.

Create a new particle system on the cube, and tweak it's settings so the particles fill the cube (something like : "Emit From : Volume", "Physics : No", "Render/Unborn : checked", "Render/Emitter : unchecked".

Select all your random objects and add them to a group (Ctrl+G). Place them on an hidden layer.

In the particle system settings, in Render, select Group, then in Dupli Group select your group.